Matting agent based on aggregated silica

ABSTRACT

The invention relates to an aggregated silica gel which has been produced from silica gel particles having a particle size of 1 to 20μ, a surface of 200 to 1000 m 2  /g and a specific pore volume of 0.4 to 2.5 ml/g and a binding agent selected from synthetic or natural phyllosilicate, pyrogenic silicon dioxide and organic polymers soluble in water or dispersible in water. Aggregation takes place by spray-drying a suspension having 1 to 25% solids. The product is suitable as a matting agent for coatings, as an antiblocking agent for polymer films and as a beer clarifying agent.

Modern matting agents for coatings consist of highly porous, amorphoussilicon dioxide which has been adjusted to a particle size which isappropriate for the required coating film thickness.

The most widely used matting agents originate from two processes:

gelation of silicic acid, washing, drying, milling

precipitation of silicic acid in such a way that a "reinforced"structure is formed, washing, drying, milling.

The best representatives of both processes show a high mattingefficiency coupled with low thickening, good dispersibility and lacquerfilm transparency (clarity). SYLOID ED5, which is manufactured by GraceGmbH, Worms, Germany, is a typical representative of the first class andis used throughout this description as a standard for the state of theart.

Another type of matting agent has also been developed which exhibits anincreased matting efficiency compared with the two aforementionedclasses. This is attained by agglomerating in various ways a pyrolytic(pyrogenic, fumed) silicon dioxide (silica) (pyrolytic silicon dioxideis obtained by flame hydrolysis of SiCl₄). This type of product (e.g. TS100 from Degussa) exhibits a matting efficiency which is increased by 10to 25% compared with the first two types. However, it is a majordisadvantage of this type of product that it increases the viscosity ofthe paint (lacquer) in which it is used, even at the lowerconcentrations which are required in order to obtain the same gloss aswith the other products. This increase in viscosity makes processing ofthe paint more difficult. A further disadvantage is the reduceddispersibility.

DE 12 09 108 describes the spray-drying of an aqueous suspensioncontaining 20 to 60% pyrolytic silicon dioxide in order to obtain aspherical product having an average particle size of approx. 200 μm.

DE 24 14 478 describes the production of a matting agent by aggregationof powdery pyrolytic silicon dioxide. The silicon dioxide is wetted byadding approx. 5% of water, based on silicon dioxide, and then dried inan oven or an air-jet mill. The surface of the powder is coated onlywith a very thin (monomolecular) layer because of low moistureabsorption. During subsequent evaporation, aggregation takes place whichleads to a product which has a relatively poor particle strength.

DE 28 31 561 describes the production of a catalyst support byspray-drying a suspension of pyrolytic silicon dioxide having aconcentration of 10 to 33% and optionally by adding ammonia. Theparticle size is 40 to 80 μm.

EP 0 341 383 describes a precipitated silicon dioxide with a high"structure", expressed as dibutyl phthalate (DBP) number of 3.0 to 4.0,which corresponds to a high pore volume. In the described process aspray-dryer is used. Given as a possible application is that as mattingagents in paints (lacquers).

DE 41 32 230 describes the use of large particles (15 to 100 μm) whichwere obtained by spray-drying a suspension of milled silicon dioxidehydrogel. These particles are used as texturing aid for coatings. Theused additives have the purpose of producing particles which do not formsolid deposits during sedimentation in the liquid paint (lacquer) as aresult of standing for a relatively long time. They do not have bindingproperties. The stability of aggregates decreases with use.

It is the object of the present invention to obtain a matting agentwhich has an efficiency which is 25% greater than that of products knownfrom the prior art, without exhibiting a correspondingly greaterthickening at the same gloss. Other important properties such asdispersibility, surface roughness and transparency (clarity) of theresulting hardened film are not to be negatively impaired either.

This object is achieved by an aggregated silica gel which has beenproduced from silica gel particles (particle size: 1 to 20μ, surface:200 to 1000 m² /g, specific pore volume: 0.4 to 2.5 ml/g) and bindingagent selected from synthetic or natural layered silicate(phyllosilicate), pyrogenic silicon dioxide and organic polymers solublein water or dispersible in water.

According to the invention a micronized silica gel having a pore volumeof 0.4 to 2.0 ml/g, preferably 1.5 to 1.9 ml/g and in particular 1.6ml/g, a particle size of 2 to 6 μm, preferably 3 to 4 μm and inparticular 4 μm (light diffraction method; Helos Particle Size Analyzer,Sympatec GmbH, Clausthal-Zellerfeld, Germany) and a surface of 200 to1000 m² /g, preferably 300 to 450 m² /g is suspended in water. A bindingagent selected from synthetic or natural layered silicate(phyllosilicate), pyrogenic silicon dioxide and organic polymers solublein water or dispersible in water is dispersed in this suspension using adissolver for agitation. A surfactant can be added in order to reducethe viscosity and to permit higher solids concentrations. Aggregationtakes place in a spray-dryer (Niro, Copenhagen, Type Minor 9). Thesuspension is atomized in a two-component nozzle (suspension throughput;2 l/h, delivery pressure of the suspension: 500 mm water column,pressure of the compressed (sprayed) air: 2.5 bar above the ambientpressure, quantity of air: 20 m³ /h). The water of the formed suspensiondroplets is evaporated in a hot (inlet temperature: 350° C.) countercurrent air stream. The remaining solid aggregates are removed from theair stream using a cyclone followed by a filter. The coarse particlesare then removed from the powder thus formed by screening(classification) in a sifter (Alpine Model MZR). The presence of thiscoarse fraction (approx. 10% of the total powder) would give the surfaceof the paint (lacquer) film a rough undesirable appearance.

When the binding agent is a synthetic or natural phyllosilicate, it ispreferably used in a ratio of 1:99 to 25:75 silicate:silica gel. Whenthe binding agent is a pyrolytic silicon dioxide, it is preferably usedin a ratio of 5:95 to 50:50 pyrolytic silicon dioxide:silica gel. Whenthe binding agent is selected from xanthane, carboxymethyl cellulose andpolyacrylates, it is preferably used in a ratio of 0.5:99.5 to 15:85organic polymer:silica gel.

Another possibility is to choose the spray conditions such thatinitially larger aggregates are formed. These are then adjusted to thedesired size by milling, followed by classificating (screening). Millingand classificating (screening) can take place at the same time in a jetmill or in separate devices (mill, sifter). The advantage of the desiredpore volume increase is retained by this process.

The product according to the invention exhibits a significantly improvedmatting efficiency compared with the silica gel starting product. Thisis a result of the additional pore volume (approx. 0.3 to 0.6 ml/g)between the particles of the aggregate (interaggregate volume). Theparticles themselves have their own internal pore volume.

A certain stability of the aggregate is necessary in order to withstandshear forces during dispersion of the matting agent into the paint. Acomparison of the product according to the invention with a standardsilicon dioxide matting agent (SYLOID ED5) gives comparable results inthis respect (Table 5).

The function of the binding agent is to stabilize the aggregates.Although it is possible to aggregate the micronized silicon dioxidewithout binding agent, the stability of this type of aggregates is notsufficient (Table 5).

An optimum stability is achieved with 1 to 25%, preferably 8 to 12% of alayered silicate (phyllosilicate) such as Optigel SH from Sudchemie. Inaddition to the preferred synthetic silicate, natural (montmorillonite,hectorite) and synthetic layered silicates (such as laponite), pyrogenic(fumed) silicon dioxide and also organic polymers soluble in water ordispersible in water, such as xanthane, carboxymethyl cellulose (CMC)and polyacrylates are effective.

The thickening power of the matting agent according to the invention islower than that of the standard SYLOID ED5 when compared atconcentrations which are required to achieve the same gloss. This issurprising because in general a higher matting efficiency is associatedwith a higher thickening power (at the same concentrations), and, whencomparing two matting agents at concentrations which lead to the samegloss, an at least equal thickening effect is expected. It is assumedthat reasons for the lower viscosity are the spherical shape and thenarrow distribution of the particles having a steepness of 0.7 to 1.0(ED5 has a steepness of 1.2 to 1.45). The steepness of the particle sizedistribution is defined as ##EQU1## (d_(v),x are defined in DIN 66141).

The other effects of the silicon dioxide in the paint (lacquer) arecomparable with those of the standard matting agent.

In order to improve the sedimentation properties of the productsaccording to the invention, it can be treated with a wax selected fromthe classes of polyethylene waxes or mineral waxes and modificationsthereof.

The products according to the invention can also be used asanti-blocking agents for polymeric films, as beer clarification agents,as thickening agents or abrasives in tooth pastes, in coatings forpaper, as catalyst supports and for refining of edible oil.

EXAMPLES Example 1

(Product according to the invention)

500 g SYLOID 244 (surface¹ : 410 m² /g, specific pore volume² : 1.58ml/g, particle size according to light diffraction method (Helos): 3.9μm) were suspended in 5 l deionized water. 50 g Optigel SH fromSudchemie were added and a sodium hydroxide solution was used to adjustthe pH value to 9. The suspension was then dispersed for 15 minutesusing a 50 mm diameter dissolver blade at 2800 rpm. The suspension wasleft to stand undisturbed overnight. It was then stirred again at 2800rpm for 5 minutes. Finally, coarse particles were removed by wetscreening through a 100 μm mesh. The suspension was atomized (sprayed)using a two-component nozzle in which the drops were generated usingcompressed air (2.5 bar above atmospheric). An air stream entering inthe spray direction having an inlet temperature of 350° C. was used toevaporate the water. The dried material was separated in a cyclone. Themedian of the volume-related particle size distribution (average volumeequivalent particle size diameter) d_(v),50 was adjusted by the pressureof the sprayed air to 10 to 14 μm (light diffraction method; Helos).

The aggregates were then classified in a dynamic classifier (sifter)Alpin model MZR 100. The particle size was adjusted to a value of 9 to12 μm by controlling the speed of rotation of the classifier (sifter).

This product was dispersed in a nitrocellulose paint (lacquer) andcompared with the standard, SYLOID ED5. The results are given inTable 1. The amount of matting agent required to obtain a gloss level of30 (angle of incidence 60°) was 24% lower in the case of the productaccording to the invention.

The material of Example 1 was also used to matt an alkyd paint (lacquer)(see Table 4). It was compared with the standard, SYLOID ED5, and withTS 100, which is one of the most efficient matting agents on the marketand was produced by aggregating pyrolytic silicon dioxide. The mattingefficiency of the product according to the invention was 18% better thanthat of the standard and 7% better than that of TS 100. More importantlyhowever, the viscosity of the wet (liquid) paint (lacquer) whichcontained the product of Example 1 was equal to that of the standardSYLOID ED5, but was significantly lower than the viscosity which ariseswhen using TS 100.

Example 2

(Agaregation without binding agent)

500 g SYLOID 244 (the same product as in Example 1) was treated exactlyas in Example 1, without adding a binding agent, and spray dried underthe same conditions as in Example 1. The results are given in Table 2.The efficiency of the product was not better than that of the standardSYLOID ED5.

Example 3

(Aggregation with organic binding agent)

3 l of water were heated to 80° C. and gently agitazed and 12 g xanthenewere added in small portions. The hot solution was allowed to cool downafter the organic thickening agent had completely dissolved. 398 gSYLOID 244 (the same product as in Example 1) were then added, and themixture was treated as in Example 1 and spray dried under the sameconditions as in Example 1. The results are given in Table 3. Theefficiency was 13% better than that of the standard SYLOID ED5.

Example 4

(Aggregation with pyrolytic silicon dioxide)

287 g SYLOID 244 (the same product as in Example 1) were suspended in 3l water. 123 g pyrolytic silicon dioxide (Aerosil 200) were added. Thesuspension was adjusted to a pH value of 9 by adding sodium hydroxideand spray dried under the same conditions as in Example 1. The resultsare given in Table 3. the efficiency was 16% better than that of thestandard SYLOID ED5.

Example 5

(improved dispersibility)

10 g of the product produced in Example 1 were filled into a plastic bagmeasuring 10 cm×10 cm. A 40 kg weight was placed on it for 30 minutes. Asample of the standard SYLOID ED5 was treated in the same way. Thedispersibility of the two samples was compared using a standardizeddispersibility test (the product is dispersed for 40 seconds in anitrocellulose paint (lacquer) using a paint shaker (Red Devil); thenumber of undispersed agglomerates in the dried paint film is used forcharacterizing the dispersibility). The film which contained thestandard matting agent SYLOID ED5 was full of visible agglomerates,whilst the film which was matted with the product according to theinvention was almost free of agglomerates. This is a significantimprovement because SYLOID ED5 is regarded as one of the mostdispersible products on the market (Tables 1 and 4).

                  TABLE 1                                                         ______________________________________                                        Performance of the aggregated matting agent                                               Application test in nitrocellulose paint                                                  Standard                                              Parameter     Unit      SYLOID ED5 Example 1                                  ______________________________________                                        Particle size μm     8.8        9.2                                        [d.sub.v,50 ] (1)                                                             Matting agent required for                                                                  % wt./wt.  0.62       0.47                                      30 gloss units (60° angle                                              of incidence) (2)                                                             Matting agent required for                                                                  % wt./wt.  0.72       0.45                                      40 gloss units (85° angle                                              of incidence) (2)                                                             Viscosity of the paint                                                                      mPa · s                                                                        86         83                                         with 1% matting agent (3)                                                     Dispersibility (4)      good       good                                       (low shear)                                                                   Surface roughness Ra at                                                                     μm     0.4        0.4                                        the same gloss (5)                                                            ______________________________________                                         (1) measured according to Helos; focal length 50 mm; median of the volume     distribution (DIN 66141)                                                      (2) DIN 67530                                                                 (3) DIN 53211; beaker opening 4 mm                                            (4) Method described in Example 5                                             (5) DIN 4768                                                             

                  TABLE 2                                                         ______________________________________                                        Performance of the aggregated matting agent                                               Application test in nitrocellulose paint                                                  Standard                                              Parameter     Unit      SYLOID ED5 Example 2                                  ______________________________________                                        Particle size μm     8.8        8.9                                        [d.sub.v,50 ] (1)                                                             Matting agent required for                                                                  % wt./wt.  0.71       0.72                                      30 gloss units (60° angle                                              of incidence) (2)                                                             Matting agent required for                                                                  % wt./wt.  0.76       0.84                                      40 gloss units (85° angle                                              of incidence) (2)                                                             Viscosity of the paint                                                                      mPa · S                                                                        361        372                                        with 1% matting agent (3)                                                     ______________________________________                                         (1) measured according to Helos; focal length 50 mm; median of the volume     distribution (DIN 66141)                                                      (2) DIN 67530                                                                 (3) DIN 53211; beaker opening 4 mm                                       

                  TABLE 3                                                         ______________________________________                                        Performance of the aggregated matting agent                                               Application test in nitrocellulose paint                                                 Standard                                                                      SYLOID   Exam-                                         Parameter     Unit     ED5      ple 3 Example 4                               ______________________________________                                        Particle size μm    8.8      8.9   9.3                                     [d.sub.v,50 ] (1)                                                             Matting agent required for                                                                  % wt./wt.                                                                              0.61     0.53  0.51                                    30 gloss units (60° angle                                              of incidence) (2)                                                             required for 40 gloss units                                                                 % wt./wt.                                                                              0.65     0.60  0.47                                    (85° angle of incidence)                                               (2)                                                                           Viscosity of the paint with                                                                 mPa · S                                                                       357      417   402                                     1% matting agent (3)                                                          Surface roughness Ra at the                                                                 μm    0.65     0.60  0.62                                    same gloss (4)                                                                ______________________________________                                         (1) measured according to Helos; focal length 50 mm; median of the volume     distribution (DIN 66141)                                                      (2) DIN 67530                                                                 (3) DIN 53211; beaker opening 4 mm                                            (4) DIN 4768                                                             

                  TABLE 4                                                         ______________________________________                                        Performance of the aggregated matting agent                                               Application test in alkyd paint                                                          Standard                                                                      SYLOID                                                 Parameter     Unit     ED5      Example 1                                                                            TS 100                                 ______________________________________                                        Particle size μm    8.8      9.2    9.1                                    [d.sub.v,50 ] (1)                                                             Matting agent required for                                                                  % wt./wt.                                                                              2.25     1.85   2.00                                   30 gloss units (60° angle of                                           incidence) (2)                                                                Viscosity of the paint with                                                                 mPa · S                                                                       111      107    142                                    1% matting agent (3)                                                          Dispersibility (4)     good     good   poor                                   (low shear)                                                                   Surface roughness Ra at the                                                                 μm    0.3      0.35   0.3                                    same gloss (5)                                                                ______________________________________                                         (1) measured according to Helos; focal length 50 mm; median of the volume     distribution (DIN 66141)                                                      (2) DIN 67530                                                                 (3) DIN 53211; beaker opening 4 mm                                            (4) Method described in Example 5                                             (5) DIN 4768                                                             

                  TABLE 5                                                         ______________________________________                                        Characterization of the particle stability                                    Particle size measurement according to different dispersion methods in        the                                                                           original paint (Example 1)                                                    The dispersed sample is diluted prior to measurement using MIBK               (methylisobutyl ketone)                                                                Median after                                                                  dispersion in                                                                             Median after                                                                             Stability index                                        the dissolution                                                                           dispersion in                                                                            Ratio of column 2/                            Product  apparatus μm                                                                           the ball mill                                                                            column 1                                      ______________________________________                                        ED5      8.85        8.25       0.93                                          ED5      8.83        8.32       0.94                                          Example 1                                                                              9.24        8.8        0.95                                                   8.58        8.13       0.95                                          Example 2                                                                              9.21        7.56       0.82                                          (no binding                                                                            7.79        6.43       0.83                                          agent)                                                                        ______________________________________                                    

We claim:
 1. Aggregated silica gel which has been produced from silicagel particles having a particle size of 1 to 20 μm, a surface area of200 to 1000 m² /g and a specific pore volume of 0.4 to 2.5 ml/g whereinthe aggregated silica gel comprises silica gel bound by a binding agentselected from the group consisting of synthetic or natural layeredsilicate, pyrolytic silicon dioxide and organic polymers soluble inwater or dispersible in water selected from xanthane, carboxymethylcellulose and polyacrylate.
 2. Aggregated silica gel according to claim1 in which the binding agent is a synthetic or natural phyllosilicatewhich is used in a ratio of 1:99 to 25:75 silicate:silica gel. 3.Aggregated silica gel according to claim 1 in which the binding agent isa pyrolytic silicon dioxide which is used in a ratio of 5:95 to 50:50pyrolytic silicon dioxide:silica gel.
 4. Aggregated silica gel accordingto claim 1 in which the binding agent is selected from xanthane,carboxymethyl cellulose and polyacrylates, which is used in a ratio of0.5:99.5 to 15:85 organic polymer:silica gel.
 5. Aggregated silica gelaccording to claim 1 which has been treated with 1 to 20% of a waxselected from the group consisting of polyethylene waxes, modifiedpolyethylene waxes, mineral waxes, modified mineral waxes, and mixturesthereof.
 6. Aggregated silica gel according to claim 2 which has beentreated with 1 to 20% of a wax selected from the group consisting ofpolyethylene waxes, modified polyethylene waxes, mineral waxes, modifiedmineral waxes, and mixtures thereof.
 7. Aggregated silica gel accordingto claim 3 which has been treated with 1 to 20% of a wax selected fromthe group consisting of polyethylene waxes, modified polyethylene waxes,mineral waxes, modified mineral waxes, and mixtures thereof. 8.Aggregated silica gel according to claim 4 which has been treated with 1to 20% of a wax selected from the group consisting of polyethylenewaxes, modified polyethylene waxes, mineral waxes, modified mineralwaxes, and mixtures thereof.
 9. Method for the production of anaggregated silica gel comprising aggregating silica gel particles havinga particle size of 1 to 20 μm, a surface area of 200 to 1000 m² /g, anda specific pore volume of 0.4 to 2.5 ml/q, wherein the aggregation takesplace with a binding agent in a spray-dryer using a suspensioncontaining 1 to 25% solids.
 10. Method according to claim 9, wherein thespray-dried solids are subsequently air-classified.
 11. Method accordingto claim 9, wherein the suspension to be spray-dried is adjusted to a pHvalue of 8 to 10.5.
 12. Method according to claim 9, wherein a waxemulsion is added to the suspension prior to the spray-drying. 13.Method according to claim 9, wherein the wax emulsion is sprayed ontothe spray-dried product and is subsequently dried.
 14. Method accordingto claim 9 wherein the binding agent is a synthetic or naturalphyllosilicate which is used in a ratio of 1:99 to 25:75 silicate:silicagel.
 15. Method according to claim 9 wherein the binding agent is apyrolytic silicon dioxide which is used in a ratio of 5:95 to 50:50pyrolytic silicon dioxide:silica gel.
 16. Method according to claim 9wherein the binding agent is selected from xanthane, carboxymethylcellulose and polyacrylates, which is used in a ratio of 0.5:99.5 to15:85 organic polymer:silica gel.
 17. A method for matting a coatingcomprising adding the aggregated silica gel of claim 1 to the coating.18. A method of imparting anti-blocking to a polymer film comprisingadding the aggregated silica gel of claim
 1. 19. A method of clarifyingbeer comprising using the aggregated silica gel of claim 1.